In the art of thermal ink jet printing, various multi-cellular materials have been used to store and deliver ink to a printing head. Included among these materials are conventional polyether polyurethane foams, reticulated cellulose, polyethylene foams and melamine-formaldehyde condensate foams. See U.S. Pat. No. 4,771,295; 4,794,409; 4,306,245 and 4,929,969.
U.S. Pat. No. 4,771,295 discloses a thermal ink jet pen body that includes a controlled pore-size reticulated polyether polyurethane foam. Reticulation breaks the walls of the small membranes within the foam material creating more passages through which liquids, such as ink, may flow. Prior to insertion into the pen body, the foam is compressed to reduce the pore size and increase the foam density, thus better controlling the capillarity of the foam. The ""295 patent does not disclose particular foam compositions and does not consider potential contamination of the ink by materials extracted from the foam when the ink is held in contact with the foam for extended periods of time.
U.S. Pat. No. 5,182,579 discloses an inkjet container filled with a compressed polyurethane foam ink absorbent member. The patent notes problems caused by impurities that may be extracted from the foam and contaminate the ink. When more than 0.04% by weight per gram of ink of the impurities are present in the ink, the surface tension and other properties of the ink are affected adversely. To solve such problems, the ""579 patent proposes washing the foam with an organic polar solvent to extract the impurities from the foam before inserting the foam into an ink jet printer.
U.S. Pat. No. 5,491,501 discloses an ink delivery medium formed from a high density, fine pore, open cell polyester polyurethane foam. The patent states that prior melamine foams emit dust that can clog the ink channel, such that melamine foams must be pre-cleaned and used in conjunction with a fine filter. The patent also states that particles break away from the prior polyurethane foams when such foams are reticulated, and that these particles must be removed from the foams with a pre-cleaning step before these foams can be used in an ink jet printer. In addition, the patent notes that most foaming processes use surfactants, and that excess surfactants must be removed from the foams with a pre-cleaning step before the foams can be used in ink jet applications. Unlike the prior melamine foams, reticulated foams and foams formed using surfactants, the particular foam disclosed in the ""501 patent is stated to be made without surfactants so that it is substantially free of residual foaming surfactants and loose particles. As a result, pre-cleaning is not required.
U.S. Pat. No. 5,587,731 discloses ink retainers made with compressed, preferably reticulated, flexible polyurethane foams with cell sizes above 20 cells/inch prepared with a polymeric polyol having styrene and/or acrylonitrile grafted thereto. Using at least 20% by weight of the polymeric polyol is said to reduce the swelling of the foam when the foam is in contact with solvents present in the ink, which in turn prevents organic matter in the foam from leaching into the ink.
The prior art ink retainers made with polyurethane foams formed with conventional polyols and surfactants emit extractable material, such as low-molecular urethane, excess unreacted surfactants, excess catalysts, antioxidants and plasticizers, when in contact with ink solvents. These extractable materials and particles can leach into the ink or ink solvent and either contaminate the ink with particles that may clog the pen, or change the surface tension of the ink and the resulting print quality. The industry is searching for less expensive foams that can be used as ink retainers in conjunction with ink jet printers without significant pre-cleaning to remove extractable materials. Accordingly, one object of the present invention is to produce polyurethane foams that emit significantly less extractable materials.
According to the invention, an ink retainer suitable for use inside an ink container for an ink jet printing system is formed from a polyether polyurethane foam that is stable to retain and supply ink and produces very low solvent extractables, less than 2.0%, preferably less than 1.5%, most preferably less than 1.1%. The foam is prepared by reacting a polyether polyol with from about 50 to 80 parts by weight, based on 100 parts polyol, of a polyisocyanate component. The polyisocyanate component is one or a mixture of polymeric methylene diphenyl diisocyanates, wherein the functionality is from 2.1 to 2.5 and the % NCO is from 28 to 32%. The isocyanate index is in the range from about 103 to 120, preferably 105 to 115. The polymeric methylene diphenyl isocyanates preferably are mixtures of the 4, 4xe2x80x2 isomer, the 2, 4xe2x80x2 isomer, and/or the 2, 2xe2x80x2 isomer of methylene diisocyanate and higher molecular weight oligomers. Some toluene diisocyanate may be incorporated into the polyisocyanate component, but preferably in an amount not exceeding 17 percent by weight, based on the weight of the polyol.
The polyurethane foam so produced has foam pore sizes above 70 pores per linear inch, preferably in the range of 70 to 90 pores per linear inch (measured by pressure drop techniques). The foam is reticulated, preferably by thermal reticulation, to remove substantially all foam cell membranes. The foam may also be felted, i.e., compressed between heated plates until the foam has a permanent compression set. When placed in a container for ink, the foam may be compressed at a compression ratio by volume of from 1 to 2 up to from 1 to 20. When the foam has smaller pore sizes, a lower compression ratio by volume is used, preferably 1 to 2 to 1 to 5.
Foam densities for polyurethane foams according to the invention are generally from 1.2 to 3.0 pounds per cubic foot, preferably 1.2 to 2.0 pounds per cubic foot. Higher or lower densities may also be suitable so long as the foam is stable to retain and supply ink and emits low extractables.